1. Field of the Invention
This invention relates to an optical system for producing a minute light beam in such a manner that the beam intensity is uniformly distributed throughout a minute image which is formed by condensing the light beam from a light source with the aid of a lens or the like.
2. Description of the Prior Art
As a method of recording wide-band signals such as TV signals on a recording medium, it is widely known to use a magnetic head movable relative to a magnetic recording medium such as magnetic tape or disc to effect recording on such magnetic recording medium. There is another method whereby a light beam such as a laser modulated by recording signal is thrown upon a sensitive plate or a thermally deformable substrate while the former is moved relative to the latter, thereby recording the recording signal on the substrate in the form of the projection locus of the light beam.
When using a light beam to effect recording on the substrate as described above, it is preferable that the light beam thrown upon the substrate be condensed to increase the density of the signals recorded on the substrate. The cross-sectional configuration of the thus condensed light beam may be linear or a minute circle in accordance with suitable selection of the method of modulating the light beam with the aid of recording signal. For example, where a linear beam is used, the light beam and the substrate may be moved relative to each other at a predetermined speed and in a predetermined direction while the light beam is being modulated for interception or passage in accordance with the recording signal. Thereby, a projection locus of the light beam parallel to the linear image and corresponding to the recording signal may be provided on the substrate in a belt-like track thereof having a width equal to the length of the linear image.
Also, where a beam of minute circular cross-section is used, the light beam and the substrate may be moved relative to each other at a predetermined speed while the light beam may be deflected to right and left over a predetermined width and at a speed corresponding to the frequency of the recording signal. Thereby, the continuation of sinewaves with the frequency thereof controlled by the frequency of the recording signal is formed on the substrate.
An arrangement for producing a light beam of linear cross-section will now be considered with reference to FIG. 1A of the accompanying drawings. A light source 11 such as conventional laser generator or the like generates a parallel beam 12 of circular cross-section, which is directed to the beam expander 13 for expanding the width of the parallel beam 12. The beam expander 13 produces a parallel beam 14 having a width (2r) greater than that of the previous parallel beam 12. The parallel beam 14 in turn is directed to a cylindrical lens 15, whereby the parallel beam 14 is diffused only in one direction parallel to the plane of the drawing sheet. The cylindrical lens need not always be of a negative refractive power as shown in FIG. 1A, but it may also be of a positive refractive power.
The light beam 16 so diffused only in one direction is directed to a condenser lens 17 which is a convex lens, whereby a linear image 19 may be formed on a focusing plate 18 positioned in the focal plane of the condenser lens 17.
Thus, a belt-like track corresponding to the formed linear image may be provided on the substrate by inserting in the path of the parallel beam 12 a light modulator which will prevent passage of the light through such path upon application of a voltage above a predetermined level but permit passage of the light through such path upon application of a voltage below the predetermined level, designing the light modulator so as to be driven by recording signal, forming the focusing plate 19 of a substrate comprising a sensitive plate, and moving the substrate in a direction perpendicular to the plane of the drawing sheet.
The above-described arrangement for producing a linear light beam suffers from disadvantages as well hereinafter be described.
Considering now the cross-section of the light beam, the cross-section of the light beam passed through the expander 13 (i.e. the cross-section of the parallel beam 14 in FIG. 1A taken along a plane perpendicular to the drawing sheet) is circular as shown in FIG. 2a (because the light source 11 emits a beam of circular cross-section), whereas the cross-section of the diffused light beam 16 adjacent the condenser lens 17 is elliptical as shown in FIG. 2b, and the image formed on the substrate by the light beam 16 passed through the condenser lens 17 to sensitize the substrate is not a perfect line but becomes bulged in the center portion (near the X-X' axis) as shown in FIG. 2c, although this figure is an exaggeration for providing a better understanding. This occurs because, as shown in FIG. 2b, the intensity of the light energy on Y-axis of the beam of elliptical cross-section before reaching the cylindrical lens 15 is greater toward the center axis and sharply decreased toward the periphery, as illustrated in FIG. 2d.
As a result, the linear image formed on the substrate of sensitive material undergoes an overexposure in the center portion thereof, so that such center portion becomes thicker as shown in FIG. 2c, and this is inconvenient to the provision of a very minute linear image.